238 lines
7.5 KiB
Plaintext
238 lines
7.5 KiB
Plaintext
unit imjfdctfst;
|
|
|
|
{ This file contains a fast, not so accurate integer implementation of the
|
|
forward DCT (Discrete Cosine Transform).
|
|
|
|
A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
|
|
on each column. Direct algorithms are also available, but they are
|
|
much more complex and seem not to be any faster when reduced to code.
|
|
|
|
This implementation is based on Arai, Agui, and Nakajima's algorithm for
|
|
scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
|
|
Japanese, but the algorithm is described in the Pennebaker & Mitchell
|
|
JPEG textbook (see REFERENCES section in file README). The following code
|
|
is based directly on figure 4-8 in P&M.
|
|
While an 8-point DCT cannot be done in less than 11 multiplies, it is
|
|
possible to arrange the computation so that many of the multiplies are
|
|
simple scalings of the final outputs. These multiplies can then be
|
|
folded into the multiplications or divisions by the JPEG quantization
|
|
table entries. The AA&N method leaves only 5 multiplies and 29 adds
|
|
to be done in the DCT itself.
|
|
The primary disadvantage of this method is that with fixed-point math,
|
|
accuracy is lost due to imprecise representation of the scaled
|
|
quantization values. The smaller the quantization table entry, the less
|
|
precise the scaled value, so this implementation does worse with high-
|
|
quality-setting files than with low-quality ones. }
|
|
|
|
{ Original: jfdctfst.c ; Copyright (C) 1994-1996, Thomas G. Lane. }
|
|
|
|
|
|
interface
|
|
|
|
{$I imjconfig.inc}
|
|
|
|
uses
|
|
imjmorecfg,
|
|
imjinclude,
|
|
imjpeglib,
|
|
imjdct; { Private declarations for DCT subsystem }
|
|
|
|
|
|
{ Perform the forward DCT on one block of samples. }
|
|
|
|
{GLOBAL}
|
|
procedure jpeg_fdct_ifast (var data : array of DCTELEM);
|
|
|
|
implementation
|
|
|
|
{ This module is specialized to the case DCTSIZE = 8. }
|
|
|
|
{$ifndef DCTSIZE_IS_8}
|
|
Sorry, this code only copes with 8x8 DCTs. { deliberate syntax err }
|
|
{$endif}
|
|
|
|
|
|
{ Scaling decisions are generally the same as in the LL&M algorithm;
|
|
see jfdctint.c for more details. However, we choose to descale
|
|
(right shift) multiplication products as soon as they are formed,
|
|
rather than carrying additional fractional bits into subsequent additions.
|
|
This compromises accuracy slightly, but it lets us save a few shifts.
|
|
More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
|
|
everywhere except in the multiplications proper; this saves a good deal
|
|
of work on 16-bit-int machines.
|
|
|
|
Again to save a few shifts, the intermediate results between pass 1 and
|
|
pass 2 are not upscaled, but are represented only to integral precision.
|
|
|
|
A final compromise is to represent the multiplicative constants to only
|
|
8 fractional bits, rather than 13. This saves some shifting work on some
|
|
machines, and may also reduce the cost of multiplication (since there
|
|
are fewer one-bits in the constants). }
|
|
|
|
const
|
|
CONST_BITS = 8;
|
|
const
|
|
CONST_SCALE = (INT32(1) shl CONST_BITS);
|
|
|
|
|
|
const
|
|
FIX_0_382683433 = INT32(Round(CONST_SCALE * 0.382683433)); {98}
|
|
FIX_0_541196100 = INT32(Round(CONST_SCALE * 0.541196100)); {139}
|
|
FIX_0_707106781 = INT32(Round(CONST_SCALE * 0.707106781)); {181}
|
|
FIX_1_306562965 = INT32(Round(CONST_SCALE * 1.306562965)); {334}
|
|
|
|
{ Descale and correctly round an INT32 value that's scaled by N bits.
|
|
We assume RIGHT_SHIFT rounds towards minus infinity, so adding
|
|
the fudge factor is correct for either sign of X. }
|
|
|
|
function DESCALE(x : INT32; n : int) : INT32;
|
|
var
|
|
shift_temp : INT32;
|
|
begin
|
|
{ We can gain a little more speed, with a further compromise in accuracy,
|
|
by omitting the addition in a descaling shift. This yields an incorrectly
|
|
rounded result half the time... }
|
|
{$ifndef USE_ACCURATE_ROUNDING}
|
|
shift_temp := x;
|
|
{$else}
|
|
shift_temp := x + (INT32(1) shl (n-1));
|
|
{$endif}
|
|
|
|
{$ifdef RIGHT_SHIFT_IS_UNSIGNED}
|
|
if shift_temp < 0 then
|
|
Descale := (shift_temp shr n) or ((not INT32(0)) shl (32-n))
|
|
else
|
|
{$endif}
|
|
Descale := (shift_temp shr n);
|
|
end;
|
|
|
|
{ Multiply a DCTELEM variable by an INT32 constant, and immediately
|
|
descale to yield a DCTELEM result. }
|
|
|
|
|
|
function MULTIPLY(X : DCTELEM; Y: INT32): DCTELEM;
|
|
begin
|
|
Multiply := DeScale((X) * (Y), CONST_BITS);
|
|
end;
|
|
|
|
|
|
{ Perform the forward DCT on one block of samples. }
|
|
|
|
{GLOBAL}
|
|
procedure jpeg_fdct_ifast (var data : array of DCTELEM);
|
|
type
|
|
PWorkspace = ^TWorkspace;
|
|
TWorkspace = array [0..DCTSIZE2-1] of DCTELEM;
|
|
var
|
|
tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7 : DCTELEM;
|
|
tmp10, tmp11, tmp12, tmp13 : DCTELEM;
|
|
z1, z2, z3, z4, z5, z11, z13 : DCTELEM;
|
|
dataptr : PWorkspace;
|
|
ctr : int;
|
|
{SHIFT_TEMPS}
|
|
begin
|
|
{ Pass 1: process rows. }
|
|
|
|
dataptr := PWorkspace(@data);
|
|
for ctr := DCTSIZE-1 downto 0 do
|
|
begin
|
|
tmp0 := dataptr^[0] + dataptr^[7];
|
|
tmp7 := dataptr^[0] - dataptr^[7];
|
|
tmp1 := dataptr^[1] + dataptr^[6];
|
|
tmp6 := dataptr^[1] - dataptr^[6];
|
|
tmp2 := dataptr^[2] + dataptr^[5];
|
|
tmp5 := dataptr^[2] - dataptr^[5];
|
|
tmp3 := dataptr^[3] + dataptr^[4];
|
|
tmp4 := dataptr^[3] - dataptr^[4];
|
|
|
|
{ Even part }
|
|
|
|
tmp10 := tmp0 + tmp3; { phase 2 }
|
|
tmp13 := tmp0 - tmp3;
|
|
tmp11 := tmp1 + tmp2;
|
|
tmp12 := tmp1 - tmp2;
|
|
|
|
dataptr^[0] := tmp10 + tmp11; { phase 3 }
|
|
dataptr^[4] := tmp10 - tmp11;
|
|
|
|
z1 := MULTIPLY(tmp12 + tmp13, FIX_0_707106781); { c4 }
|
|
dataptr^[2] := tmp13 + z1; { phase 5 }
|
|
dataptr^[6] := tmp13 - z1;
|
|
|
|
{ Odd part }
|
|
|
|
tmp10 := tmp4 + tmp5; { phase 2 }
|
|
tmp11 := tmp5 + tmp6;
|
|
tmp12 := tmp6 + tmp7;
|
|
|
|
{ The rotator is modified from fig 4-8 to avoid extra negations. }
|
|
z5 := MULTIPLY(tmp10 - tmp12, FIX_0_382683433); { c6 }
|
|
z2 := MULTIPLY(tmp10, FIX_0_541196100) + z5; { c2-c6 }
|
|
z4 := MULTIPLY(tmp12, FIX_1_306562965) + z5; { c2+c6 }
|
|
z3 := MULTIPLY(tmp11, FIX_0_707106781); { c4 }
|
|
|
|
z11 := tmp7 + z3; { phase 5 }
|
|
z13 := tmp7 - z3;
|
|
|
|
dataptr^[5] := z13 + z2; { phase 6 }
|
|
dataptr^[3] := z13 - z2;
|
|
dataptr^[1] := z11 + z4;
|
|
dataptr^[7] := z11 - z4;
|
|
|
|
Inc(DCTELEMPTR(dataptr), DCTSIZE); { advance pointer to next row }
|
|
end;
|
|
|
|
{ Pass 2: process columns. }
|
|
|
|
dataptr := PWorkspace(@data);
|
|
for ctr := DCTSIZE-1 downto 0 do
|
|
begin
|
|
tmp0 := dataptr^[DCTSIZE*0] + dataptr^[DCTSIZE*7];
|
|
tmp7 := dataptr^[DCTSIZE*0] - dataptr^[DCTSIZE*7];
|
|
tmp1 := dataptr^[DCTSIZE*1] + dataptr^[DCTSIZE*6];
|
|
tmp6 := dataptr^[DCTSIZE*1] - dataptr^[DCTSIZE*6];
|
|
tmp2 := dataptr^[DCTSIZE*2] + dataptr^[DCTSIZE*5];
|
|
tmp5 := dataptr^[DCTSIZE*2] - dataptr^[DCTSIZE*5];
|
|
tmp3 := dataptr^[DCTSIZE*3] + dataptr^[DCTSIZE*4];
|
|
tmp4 := dataptr^[DCTSIZE*3] - dataptr^[DCTSIZE*4];
|
|
|
|
{ Even part }
|
|
|
|
tmp10 := tmp0 + tmp3; { phase 2 }
|
|
tmp13 := tmp0 - tmp3;
|
|
tmp11 := tmp1 + tmp2;
|
|
tmp12 := tmp1 - tmp2;
|
|
|
|
dataptr^[DCTSIZE*0] := tmp10 + tmp11; { phase 3 }
|
|
dataptr^[DCTSIZE*4] := tmp10 - tmp11;
|
|
|
|
z1 := MULTIPLY(tmp12 + tmp13, FIX_0_707106781); { c4 }
|
|
dataptr^[DCTSIZE*2] := tmp13 + z1; { phase 5 }
|
|
dataptr^[DCTSIZE*6] := tmp13 - z1;
|
|
|
|
{ Odd part }
|
|
|
|
tmp10 := tmp4 + tmp5; { phase 2 }
|
|
tmp11 := tmp5 + tmp6;
|
|
tmp12 := tmp6 + tmp7;
|
|
|
|
{ The rotator is modified from fig 4-8 to avoid extra negations. }
|
|
z5 := MULTIPLY(tmp10 - tmp12, FIX_0_382683433); { c6 }
|
|
z2 := MULTIPLY(tmp10, FIX_0_541196100) + z5; { c2-c6 }
|
|
z4 := MULTIPLY(tmp12, FIX_1_306562965) + z5; { c2+c6 }
|
|
z3 := MULTIPLY(tmp11, FIX_0_707106781); { c4 }
|
|
|
|
z11 := tmp7 + z3; { phase 5 }
|
|
z13 := tmp7 - z3;
|
|
|
|
dataptr^[DCTSIZE*5] := z13 + z2; { phase 6 }
|
|
dataptr^[DCTSIZE*3] := z13 - z2;
|
|
dataptr^[DCTSIZE*1] := z11 + z4;
|
|
dataptr^[DCTSIZE*7] := z11 - z4;
|
|
|
|
Inc(DCTELEMPTR(dataptr)); { advance pointer to next column }
|
|
end;
|
|
end;
|
|
|
|
end.
|